The present invention pertains to steel reinforcement used in concrete block construction, and more particularly to a device for positioning vertical steel reinforcement bars precisely where they are designed to be located, so that the full tensile strength of the steel bar is utilized, as intended by the engineer who designs the wall.
Concrete blocks and mortar have relatively little tensile strength, and it is therefore necessary to use steel reinforcing bars to provide the necessary tensile strength; some of the bars extending vertically through the cavities in the blocks, so as to anchor the blocks to the concrete footing; and some extending horizontally along the motar joints at various levels. The vertical bars are particularly important, as they provide most of the tensile strength to prevent the wall from failing under horizontal pressure, such as the pressure exerted by soil on one side of the wall. For most economical design, the engineer locates the bar as far as possible from the neutral axis of the wall toward the tension side of the wall, while still allowing a certain minimum amount of space between the bar and the inner surface of the block cavity to allow concrete grout to flow around the bar so as to completely embed it. This is done by specifying a distance "d" from the outer surface of the block on the compression side of the wall to the center of the vertical steel bar. The larger the distance "d", the smaller the required steel area. If the actual "d" is smaller than the design "d", then the actual stress on the steel will be greater than the calculated stress, with the consequent potential for overstressing the steel.
In practice, it is difficult and, in fact, almost impossible to keep the vertical reinforcing bar at the precise distance "d" relative to the outer surface of the blocks from the top of the wall to its bottom as the grout is being poured. As a result, the bar may drift or bend away from the inner surface of the block toward the neutral axis of the wall, where the bar is subject to being overstressed. To compensate for uncertainty as to the actual distance "d", design engineers customarily use only 50% of the tensile strength of the steel in their calculations if there is not to be continual inspection of the steel on the job. This results in the use of oversize steel bars. Even when oversize steel reinforcing bars are specified, there is some danger that the bars may become displaced far enough from their design location to make them relatively ineffective, resulting in overstress in the masonry. Specifying oversize reinforcing bars is expensive, and adds greatly to the cost of the wall.